Add API key authentication middleware support

Introduced `ApiKeyAuthOptions` for API key configuration,
including a `Key` property and `SectionName` for binding.
Updated `ApiKeyAuthMiddleware` to validate the `X-Api-Key`
header against the configured API key.

Added conditional middleware application in `Program.cs`
based on the presence of a configured API key. Bound the
`ApiKey` section in `appsettings.json` and
`appsettings.Development.json` to store the API key, with
a default value of `null`. This improves security and
centralizes API key management.

src/presentation/DigitalData.TaskFlow.API/appsettings.Development.json

@@ -10,6 +10,9 @@
       }
     }
   },
+  "ApiKey": {
+    "Key": null
+  },
   "Logging": {
     "LogLevel": {
       "Default": "Information",